Modern digital communication systems have sophisticated data modulation schemes to meet the increasing demand for high information rate in band-limited channels, and consequently require more advanced signal processing techniques to support the high data rates. However, more sophisticated signal processing and modulation schemes are more sensitive to distortions.
One such advanced modulation scheme is a system that expands information capacity of an existing communication system by embedding data in the co-channel signals of an existing infrastructure, described in “Expanded Information Capacity For Existing Communication Transmission Systems” by Hartson et. al. (U.S. Pat. No. 6,433,835), which has been further developed in “Adaptive Expanded Information Capacity For Communication Systems” by Long et. al. (U.S. CIP application Ser. No. 10/246,084). In this system, data signals are inserted in a co-channel analog TV signal, with power levels lower than those of the co-channel signals, and are processed by devices originally designed for the co-channel signals.
The low-power data signals added to dominating co-channel signals face many obstacles. The main obstacle, perhaps, is a non-linear distortion caused by a power amplifier (PA) designed for the co-channel signals rather than for the data signals. Therefore, the throughput of this data-over-co-channel-signal system can be seriously degraded in the presence of the PA distortion. The PA non-linear distortion problem in communication systems has been well recognized and several compensation schemes have been developed to mitigate it.
In most digital communication systems, information bits are mapped to real-valued (or complex-valued) numbers and converted to a band-limited waveform by a pulse-shaping filter. The pulse-shaped signals are then modulated to a radio frequency (RF) signal and fed to a PA for power amplification before transmission. Due to power efficiency and physical limitations of the amplifying devices, the PA often works outside of its linear region and introduces larger amplitude and phase distortion for the RF signals of large envelope amplitude than for the RF signals of small envelope amplitude—especially in the region close to the PA saturation point.
Because the envelope amplitude of the RF signals is the amplitude of the pulse-shaped signals, the PA non-linear distortion can be modeled in baseband as a memory-less, time-varying amplitude-dependent amplitude distortion (AM/AM) and amplitude-dependent phase distortion (AM/PM) as a function of the amplitude of the pulse-shaped signals.
To mitigate the PA distortion, several DSP techniques have been developed, such as “Adaptive Parametric Signal Predistorter For Compensation Of Time Varying Linear And Nonlinear Amplifier Distortion” by Stonick et.al. (U.S. patent application Ser. No. 852,944). The prior art techniques compare the pulse-shaped RF converter input signals and the baseband demodulated signals from PA RF output to generate error signals and to construct a pre-distortion mapping for the RF converter input.
Some schemes first estimate the non-linear characteristics of the PA, based on the comparison error, and then invert the characteristics to obtain pre-distortion mapping, for example “Generalized Data Predistortion Using Intersymbol Interpolation,” by Karam et. al., Philips J. of Research, pp.1-22, vol. 46, no 1, 1991.
Some schemes directly minimize the comparison error by adaptively adjusting the pre-distortion mapping, for example, with a polynomial approximation as described in “Adaptive Parametric Signal Predistorter For Compensation of Time Varying Linear and Nonlinear Amplifier Distortion” by Stonick et.al. (U.S. Pat. No. 5,900,778).
Comparing baseband PA input and output is not applicable to data-over-co-channel-signal systems such as the systems proposed in “Expanded Information Capacity For Existing Communication Transmission Systems” by Hartson et. al. (U.S. Pat. No. 6,433,835) and “Adaptive expanded information capacity for communication systems” by Long et. al. (U.S. CIP application Ser. No. 10/246,084). In most cases, the co-channel signals are already compensated in their own compensation mechanism, independently of the data.
A feasible approach for this system is to directly use the mean squared error (MSE) or the bit error rate (BER) of demodulated data symbols, instead of the intermediate pulse-shaped signals. Unfortunately, one critical drawback inhibiting practical use of this approach is the difficulty of obtaining the pre-distortion mapping from the MSE of the symbol outputs. The MSE function of the symbol outputs does not allow any closed form expression for its gradient with respect to the pre-distortion mapping parameters. Furthermore, regressor vectors are often inaccessible even for the conventional numerical gradient approximations such as simplex algorithms, let alone the prohibitively expensive computational complexity of numerical approaches, such as the ones discussed in “Theory and Algorithms For Linear Optimization” by Roos et. al., John Wiley & Son, 1997.